Cystic fibrosis (CF) is the most common, lethal autosomal recessive disorder in the U.S. Novel therapeutic approaches to CF lung disease, the major cause of morbidity and mortality, are clearly needed. Published studies indicate significant heritability of lung disease severity in CF, independent of CFTR genotype. To search for genes modifying CF lung disease, we performed a genome-wide association scan in one cohort of CF patients, with replication of top candidates in an independent cohort. Using this approach, genetic variation in IFRD1 was identified and replicated as a modifier of lung disease severity in CF. IFRD1 is a transcriptional co-regulator that acts in a histone deacetylase (HDAC)-dependent manner. While expressed in numerous cell types, IFRD1 appears to be expressed most highly in neutrophils (PMNs). Our data indicate that PMN differentiation is associated with upregulation of IFRD1 expression; and that PMNs (but not macrophages) from Ifrd1-/- mice have impaired effector function. The inflammatory response in the CF airway is persistently neutrophilic; in turn, the products of activated PMNs appear to be responsible for airway destruction in CF. Our data demonstrate that, after airway infection with P. aeruginosa, Ifrd1-/- mice exhibit significantly slower bacterial clearance; but also significantly less neutrophilic inflammation and disease. This phenotype is strictly dependent upon hematopoietic cell expression of IFRD1. Further, HDAC inhibition leads to specific blunting of airway inflammatory responses in wild type, not Ifrd1-/- mice, indicating that IFRD1 acts in an HDAC-dependent fashion to regulate airway inflammation. Bone marrow transfer techniques and intracellular analysis of cytokine production localized these effects to PMNs. Finally, analysis of PMNs from normal human donors has revealed significant association of IFRD1 polymorphisms with quantitative measures of PMN effector function. Together, these data suggest the inter-related hypotheses that underlie this proposal: (a) IFRD1 is a modifier gene for CF lung disease; (b) IFRD1 modulates the pathogenesis of airway disease in CF through regulation of PMN effector function; and (c) polymorphisms in IFRD1 alter PMN function. The studies in this proposal will define the biological consequences and cellular and molecular mechanisms underlying IFRD1- mediated modulation of PMN function, as well as identify the causal variants in IFRD1 that modify the expression of CF lung disease.